Skip to main content
Biology LibreTexts

46: Microbial Foods

Objectives

  • Observe the role of bacteria in fermentation of yogurt and mead

YOGURT 

This lactic acid fermentation was probably a way for people to preserve the milk, yet kill off pathogenic organisms. The low pH inhibits most microorganisms: however, the fermenting microbes are Streptococcus thermophilus and Lactobacillus vulgaricus. The resulting custard-like texture is a curd produced by the acid by-product of lactose breakdown, followed by the coagulation of the milk protein casein.

MEAD

Mead is a very old fermented drink. The main ingredient of any mead is honey, but there are also fruit and spices added as flavorings. The alcohol concentration varies from 8-18%. By changing the proportion of honey and water, one can vary the end product from dry and light to sweet and heavy-bodied.

We have to force the yeast to metabolize anaerobically---to ferment. The containers will have airlocks on them to cut oxygen off and keep carbon dioxide in. One sugar molecule is changed into two alcohol and two carbon dioxide molecules.

MATERIALS NEEDED

For both yogurt and mead

  • ice bath
  • thermometer
  • hotplate

For yogurt

  • 8 oz. low fat milk
  • Dannon plain yogurt with active cultures
  • spoon
  • 1 liter beaker
  • 2 Tbs. powdered skim milk
  • 500 ml graduated cylinder
  • styrofoam cup and lid

For mead

  • 6 oz. honey
  • SPICES: 1 whole clove slightly cracked, 1 slice of ginger, 1 tsp. orange zest, 1/2 cinnamon stick broken up
  • flask (Volume = 5 L)
  • glass bottle for mead fermentation + airlock (shared with class)
  • 2 L beaker
  • 1 L graduated cylinder
  • citric acid powder
  • malt powder
  • vegemite
  • bentonite powder
  • Saccharomyces yeast starter

 

THE PROCEDURES

Yogurt

The following recipe is for 2containers of yogurt of 16 oz. ounces total.

  1. Measure out 1 cup (236 ml. = 1 cup) of milk, optimally lowfat milk.
  2. Add 1 level Tbs. of powdered skim milk to the milk (adds extra protein) and stir.
  3. Bring the mixture to a boil over medium heat for 30 seconds, stirring constantly. You do not want the milk to burn to the bottom of the beaker or flask. Cool milk to around 45º C using a thermometer to check temperature.
  4. The inoculum used in class is grocery store, plain yogurt (with active cultures). Use 1 tsp. of the yogurt as your starter culture, stirring it into the milk with a glass stirring rod.
  5. Pour the mixture into 1 styrofoam cup and cover with a lid.
  6. Incubate at 37º C for 9-15 hours or until desired firmness is obtained. The lab personnel will remove the yogurt samples the following day.
  7. Place in the fridge until cool. Add honey or fruit when ready to eat.

Mead

The following recipe is for 700 ml of mead.

  1. Mix 500 ml water, cloves, ginger, orange, and cinnamon.
  2. Bring it to boil for 10 minutes and simmer for 5 minutes.
  3. Add 6 oz. of honey (pour some of the simmered water into honey container to get it all out).
  4. When adding honey to hot or boiling water, STIR CONSTANTLY!! Otherwise, the honey will go straight to the bottom of the pot, where it will caramelize and scorch. While it is cooking a foam will form which should be removed with paper towels. Take the honey solution back to boiling for a couple of minutes.
  5. Add 1/4 tsp. of bentonite to honey solution: this will help to settle out the solid material.
  6. After it is cooled to 45º C, the yeast starter can be added and the honeywater solution filled into the flask.
  7. Add 1/4 tsp. citric acid and 1/2 tsp. malt extract to the solution (vitamins and acidity is helpful for the yeast). You can also add a dab of Vegemite (adds extra vitamins for the yeast growth).
  8. To reduce oxygen, the flask will be closed with an airlock. The carbon dioxide will be let out.
  9. Bottle the mead.
  10. Usually it takes about 3 weeks to get the mead, but the time depends on the amount of yeast and the honey. After fermenting, you can pour some of the mead into a small sterile bottle (an oven for 1 hour at 350º F) so that it can AGE. Aging the mead will reduce some of the 'rough' edges and make it a more subtle wine. Otherwise, we will taste the mead after just a short fermentation for a few weeks.

Yeast Starter

The yeast, Saccharomyces, come out of the package dry and dormant. This mixture will allow the dry yeast to start to metabolize, so that they can just 'take off' when added to the honey solution. Yeast starter will be made 1 day ahead, and it will be provided to you during lab. However, in case you are interested, here is the recipe (this starter will make 7 1/2 gallons of mead).


3 cups Water

3 Tablespoons sugar

1/3 juice of a lemon yeast for 7.5 gallons of mead

All ingredients are mixed in a glass jar, shaken, the bottle closed loosely with a cloth cover, sitting over night. Next day it should be in a full ferment and can be added to your must (unfermented fresh honey solution). Each table will receive 20 ml of the yeast starter to make the 700 ml of mead.

QUESTIONS

  1. Describe how the bacteria produce the yogurt consistency---the chemical reaction, nutrient being used, end production, pH changes.
  2. The starting material of wine is usually a gruit or fruit juice of some sort, unlike mead, which uses the starting material _________.
  3. Why do you have to cool the milk to around 45o C before adding the starter cultures?

Contributors